Diodes in parallel

[madires]

You can parallel schottky diodes, but better not PN diodes, unless they are really close that you can consider then isothermal.

Schottky diodes' internal drift region resistance will increase with temperature due to reduced carrier mobility, thus current will tend to shift to the one that is cooler, balancing the temperature.

Still I see shorted Schottky diodes at the secondary side of SMPSUs with a few diodes in parallel?

[T3sl4co1l]

Although modern power FETs do suffer poor linear SOA due to hotspots as a result. Just saying.

Not as a rule.  SuperJunction types often give DC SOA, despite having higher current density than ever.  I've actually seen a few that don't, not sure if that's a new trend or I just haven't looked at enough of them.

IGBTs have even higher power density still; they largely don't have linear mode ratings at all, but I have seen a few that do, astonishingly enough.

Tim

[Siwastaja]

Still I see shorted Schottky diodes at the secondary side of SMPSUs with a few diodes in parallel?

The issue is you assuming they are dead simply because of paralleling. The corollary is, single diodes can't fail.

Actually, the diodes you see dying die due to overvoltage or overcurrent, basically exceeding the die temperature limit. In other words, the diodes are sized wrong. A single diode can be undersized, as well. If you have a single undersized diode and beef it up by paralleling two, it will do better. But it won't be twice as good.

Of course, as explained by many already, paralleling diodes is tricky because current likely isn't shared perfectly. Derating is needed, and inexperienced or "I don't care" engineer may not have any idea how much to derate. So it's one trap for the young players more. This said, in a power supply design, there are dozens of such tricky areas that require expertise. You can fail at any of them.

BTW I have never seen a failed paralleled double diode in a power supply.

[Harshadb13]

What happens when two diodes are in parallel? if this is a bad idea, why exists shotkky diodes with common cathode? (i. e. to220 package).

Well I believe, having a small resistance in series to the each of the diodes and having this configuration in parallel might help.

Please let me know if what I am speaking is wrong or has some problem

[Gyro]

As has been said, it all depends on how well they are matched. Heck, almost every diode on a IC will be made up out of loads of tiny diodes put in parallel. Same applies to FETs, in the end I can only make a tiny FET on chip, and to get bigger ones I have to put them in parallel, with multiple fingers.

Although modern power FETs do suffer poor linear SOA due to hotspots as a result. Just saying.

I'm no power-semiconductor guy, so I can't be sure, but isn't that more a matter of optimization? Most of those power FETs are targeted at switching applications, so linear-region SOA is not a primary concern in their design.

Yes, I'm not sure either. As T3sl4co1l says, you can get modern devices suitable for linear operation. I guess the devil is in fine the detail. I've rather lost track since power lateral mosfets used in linear operation. They had positive temperature coefficient and so gave pretty much perfect sharing, both across the die and between devices.

[Circlotron]

As has been said, it all depends on how well they are matched. Heck, almost every diode on a IC will be made up out of loads of tiny diodes put in parallel. Same applies to FETs, in the end I can only make a tiny FET on chip, and to get bigger ones I have to put them in parallel, with multiple fingers.

Although modern power FETs do suffer poor linear SOA due to hotspots as a result. Just saying.

I'm no power-semiconductor guy, so I can't be sure, but isn't that more a matter of optimization? Most of those power FETs are targeted at switching applications, so linear-region SOA is not a primary concern in their design.

Yes, I'm not sure either. As T3sl4co1l says, you can get modern devices suitable for linear operation. I guess the devil is in fine the detail. I've rather lost track since power lateral mosfets used in linear operation. They had positive temperature coefficient and so gave pretty much perfect sharing, both across the die and between devices.

When fully turned on the rds has a positive temp coefficient so they will current share somewhat, but if they are only partially turned on the gate threshold voltage has a negative temp coefficient so the hotter mosfet will dig in harder. Worse still, the hotter part of the die will hog the current, as has been mentioned, also because of gate threshold tempco.